1. Field of the Invention
The present invention relates to a fully differential amplifier device, in particular of the type with output-common-mode feedback, and to a control method thereof.
2. Description of the Related Art
As is known, the use of fully differential operational amplifiers with switched-capacitor circuits affords important advantages in several applications, especially owing to low levels of consumption and to compatibility with low supply voltages (3 V or even lower).
There are, however, some critical aspects, which can cause serious problems in certain circumstances. In particular, it is known that fully differential operational amplifiers employ output-common-mode-feedback (OCMFB) networks, which fix common-mode working points not only for the inputs, but also for the outputs. The OCMFB networks can be of the switched-capacitor type. In this case, common-mode control envisages a first step (reset), in which a capacitor for each output is charged to a pre-determined voltage, and a second step (active or “sensing” step), in which the charged capacitors are connected to the operational amplifier so that the pre-determined voltage is applied between the outputs and a common-mode-feedback node. In addition, in order to correctly balance the operational amplifier, its inputs and outputs are shorted in the reset step. During power-on or exit from low-consumption wait states (“Power Down”, “Deep Power Down”), the short circuit between the inputs and the outputs of the operational amplifier is critical. In fact, it is possible that the outputs of the operational amplifier will be at a zero voltage or in any case at a voltage much lower than the lowest available supply source. Since in the reset step the inputs are forced to the same voltage as the outputs, it might happen that the operational amplifier does not switch on. Obviously, in this situation, the operational amplifier is not able to supply current to the outputs, which remain at a low voltage. Furthermore, the OCMFB network tends disadvantageously to force the feedback node to a negative voltage (lower than the ground voltage or than the lowest available supply source). Switching-on of the operational amplifier can be triggered by random fluctuations, due, for example, to the presence of noise, or by uncontrolled variations of the electrical quantities present. However, the evolution of the state of the operational amplifier is indeterminate: according to random variations in the specific operating conditions, it may happen that the switching-on or restoring time of the operational amplifier is very long, or else even that return to normal operating conditions fails.
In order to solve the problem described, it has been proposed to include in the operational amplifier further control circuitry, which intervenes only in particular operating conditions. The solution is not altogether satisfactory because it requires a considerable occupation of area, given a minimum use in terms of time.